1. Technical Field of the Invention
The present invention relates to an active matrix type liquid crystal display employing an inverted staggered type thin-film transistor (hereinafter referred to as “TFT”) as an active element and having a color layer on a wafer side where the TFT is mounted, and, in particular, relates to alignment marks which serve as an alignment reference in each production step and a manufacturing method for the same.
2. Description of the Related Art
FIGS. 1A and 1B show a first embodiment of the prior art, that is, schematic views of a channel etch type TFT of an active matrix wafer in a liquid crystal display. FIG. 1A shows a plan view of one picture element, FIG. 1B shows a section of a region of the TFT along the cut line I—I of FIG. 1A, and FIGS. 2A and 2B show sections of a terminal portion. In FIG. 1B, a gate electrode 42a is formed on a transparent insulated wafer 41 and thereon, a gate insulating film 43 is formed to cover them. Further thereon, a semiconductor layer 44 is formed so as to overlap the gate electrode 42a, and a source electrode 46a and drain electrode 47 distant on the central part of the semiconductor layer is connected to the semiconductor layer 44 via an ohmic contact layer 45. The ohmic contact layer between the source electrode 46a and drain electrode 47 is removed by etching and the ohmic contact layer 45 is formed only between the source electrode 46a, the drain electrode 47 and semiconductor layer 44. Further, a passivation film 48 is formed so as to cover them. On the passivation film 48, a transparent conductive film to be a picture element electrode 49 is connected to the drain electrode 47 via a contact through hole 51 penetrating through the passivation film 48. A switching signal is inputted to the TFT through a gate wiring 42b and the source electrode 42a, and an image signal is inputted through a source wiring 46b and the source electrode 46a, whereby a picture element electrode 49 is charged.
Then, a manufacturing method for the active matrix wafer shown in FIGS. 1A, 1B, 2A and 2B is explained with reference to FIGS. 3A to 3C and 4A and 4B. Here, the TFT portion is shown on the left side of FIGS. 3A to 3C and an alignment portion, which is used for alignment with a mask in an exposure device in each photolithography step, is shown on the right side of FIGS. 3A to 3C. The alignment portion is provided, as shown in FIG. 5A, on the outside of a picture element display area 65 of the active matrix wafer. FIG. 5B is an enlarged plan view of the alignment portion and FIG. 5C is a section thereof.
As shown in FIG. 3A, on the transparent insulated wafer 41 made of glass, etc., a conductive layer made from Al, Mo, and Cr, etc. is deposited to be 100 to 400 nm in thickness by sputtering and a first patterning step is performed such that, through a photolithography step, gate wiring (not illustrated), gate electrodes 2a, and gate terminals (not illustrated) connected to an external signal processing wafer for display are formed. Here, gate alignment marks 63a used for overlapping with gate wiring and gate electrodes in the following step are formed outside the display area by the same layer. Then, as shown in FIG. 3B, a second patterning step is performed such that a gate insulating film 43 made of a silicon nitride film, etc. and a semiconductor layer 44 made of amorphous silicon, and an ohmic contact layer 45 made of n+ amorphous silicon are laminated to be approximately 400 nm, 300 nm, and 50 nm in thickness in sequence, respectively, and the semiconductor layer 44 and ohmic contact layer 45 are collectively patterned. Herein, for patterning, as shown in FIG. 3B, alignment between a mask 61 and an active matrix wafer 50c is necessary in the exposure device.
The alignment is performed, as shown in FIG. 3B, as follows; gate alignment marks 63a, formed in the first patterning step where the gate electrode 42a, etc. is formed, are used which are aligned with mask side alignment marks 62 formed on a mask 61. For aligning the alignment marks, as shown in FIG. 6A, respective alignment marks formed on the active matrix wafer 50c and mask 61 are read by means of a laser beam, whereby the mask side alignment marks 62 and the active matrix wafer side alignment marks 63 are aligned. At this time, for reading the alignment marks, as shown in FIG. 6B, an exposure alignment laser 66 is irradiated on alignment marks 60 through a transparent film 67 and a light reflected from alignment marks 60 or, as shown in FIG. 6C, a light diffracted from the step portion due to the alignment marks 60 are read. When reading is performed by means of the reflected light, it is necessary that the alignment marks are formed of a metal which reflects the laser beam and if the marks have a film thereon, its material does not absorb the reflected light. Also, when reading is performed by means of the diffracted light based on the step portion, the alignment marks have no restriction in material and if the marks have a film thereon, as shown in FIG. 6C, it may be a non-transparent film 68, and further, it is necessary that the step portion of the alignment marks are not flattened due to the film material and film thickness.
Then, a third patterning step is performed, as shown in FIG. 3c, in that Mo and Cr, etc. are deposited to be 100 to 200 nm in thickness so as to cover the gate insulating film 43 and ohmic contact layer 45 by sputtering and thereon, source electrodes 46a, source wiring 46b, drain electrodes 47, and lower electrodes 47d (FIG. 2B) of data terminals 47a connected to an external signal processing wafer for display are formed by a photolithography step, as shown in FIG. 4A. Here, in an exposure step, the mask 61 and active matrix wafer 50c are aligned, as shown in FIG. 3C, by means of the gate alignment marks 63a formed in the first patterning step. Further, as shown in FIG. 4A, drain layer alignment marks 63b are formed of a drain metal material at the same time in the third patterning step. After the third patterning step, the unnecessary ohmic contact layer 45 on the area other than under the source electrode 46a and drain electrode 47, which serves as a channel portion of the TFT, is removed.
Thereafter, a fourth patterning step is performed, as shown in FIG. 4A, in that, a passivation film 48 made of an inorganic film such as a silicon nitride film is formed to be 100 to 200 nm in thickness by the plasma CVD method so as to cover the back channel of the TFT, that is, the source electrode 46a, source wiring 46b, the drain electrode 47, and a lower electrode 47d of a data terminal 47a, a contact through hole 51 for making a contact with the drain electrode 47 and picture element electrode 49 is formed, and unnecessary gate insulating film 43 on the lower electrode 47d (FIG. 2B) of the data terminal 47a and unnecessary gate insulating film 43 and passivation film 48 on the lower electrode (FIG. 2A) of the gate terminal 42c are removed. Herein, alignment between the mask 61 and active matrix wafer 50c in an exposure step is performed, as shown in FIG. 4A, by means of the drain layer alignment marks 63b formed in the third patterning step.
Lastly, as shown in FIG. 4B, a fifth patterning step is performed such that a transparent conductive film 149 to be a picture element electrode is formed by sputtering. Herein, alignment between the mask 61 and active matrix wafer 50c in an exposure step is performed, as shown in FIG. 4B, by means of the drain layer alignment marks 63b formed in the third patterning step.
An active matrix wafer shown in FIG. 1B is produced by such a manufacturing method through five patterning steps, and therefore, the production process is significantly shortened. The active matrix wafer is used and combined with color filters and another wafer having electrodes, liquid crystal is sandwiched between two wafers, thereby composing a liquid crystal display.
However, in this active matrix wafer, when shown as a plan view of FIG. 1A, light leaks between the gate wiring 42b and source wiring 46b and picture element electrode 49, and therefore, it is necessary to be shaded by a black matrix provided on a color filter wafer. In this case, when lamination accuracy of the color filter wafer and active matrix wafer is taken into consideration, a large shaded area of the black matrix has to be secured, and therefore, there are problems in that the aperture ratio of the liquid crystal display is decreased and backlight is not effectively utilized.
With respect to the above problems, as a means for improving aperture ratio, a method, in which a color filter wafer is formed on an active matrix wafer (a CF on TFT structure), has been suggested, for example, in Japanese Laid-open Patent Publication No. 39292 of 1998, which is a second prior art. For producing such a structure, when conditions, etc. which have not described in the Publication are added, its actual manufacturing method is as follows.
As shown in FIG. 7B, on a passivation film 78, a dispersed pigment type photoresist black matrix is coated by spin coating and, by a photolithography step, a black matrix 85′ is formed on the gate wiring including an area where a contact hole is to be formed and on a channel etch type TFT 10a. The spin speed of a spin coater is adjusted so that the film thickness becomes approximately 1.5 μm. Here, in an exposure step, alignment between a mask 91 and an active matrix wafer 80c is performed, as shown in FIG. 10B, by means of active matrix wafer side alignment marks 93 formed in a TFT formation step.
Then, as shown in FIG. 7c, on an active matrix wafer which has been cleaned by ultraviolet light, a dispersed pigment type photosensitive red color resist is coated to be approximately 1.2 μm in thickness by spin coating and a red filter 83a′ is formed into a predetermined pattern by a photolithography step. Here, in an exposure step, alignment between a mask 91 and an active matrix wafer 80c is performed, as shown in FIG. 10C, by means of active matrix wafer side alignment marks 93 formed in a TFT formation step.
Then, as shown in FIG. 8A, for the purpose of forming a green filter, on an active matrix wafer which has been cleaned by ultraviolet light, a dispersed pigment type photosensitive green color resist is coated to be approximately 1.2 μm in thickness by spin coating and a green filter 83b′ is formed into a predetermined pattern by a photolithography step. Here, in an exposure step, alignment between a mask 91 and an active matrix wafer 80c is performed, as shown in FIG. 11A, by means of active matrix wafer side alignment marks 93 formed in a TFT formation step.
Then, as shown in FIG. 8B, for the purpose of forming a blue filter, on an active matrix wafer which has been cleaned by ultraviolet light, a dispersed pigment type photosensitive blue color resist is coated to be approximately 1.2 μm in thickness by spin coating and a blue filter 83c is formed into a predetermined pattern by a photolithography step. Here, in an exposure step, alignment between a mask 91 and an active matrix wafer 80c is performed, as shown in FIG. 11B, by means of active matrix wafer side alignment marks 93 formed in a TFT formation step.
Then, as shown in FIG. 3c, on a TFT wafer on which the black matrix 85′, red filter 83a′, green filter 83b′, and blue filter 83c′ are formed, an overcoat layer 84 for flattening the TFT wafer is formed to be approximately 3 μm in thickness. A photosensitive acrylic resin is used and coated as the overcoat layer by spin coating, and then an aperture is provided for a contact through hole portion 81 on the overcoat layer. Since alignment between a mask and active matrix in an exposure step is performed in a similar manner to that of the foregoing steps, and it is likewise in the following steps, the description thereof will be omitted (not illustrated).
Then, as shown in FIG. 9A, after a novolac photoresist 87 is coated on the overcoat layer 84 which is then patterned, the black matrix at the contact through hole portion is removed by means of the novolac photoresist 87 as its mask by a dry etching.
Then, as shown in FIG. 9B, an aperture is provided on the passivation film 78 for the contact through hole by dry etching, and the complete aperture for the contact through hole is provided. Lastly, a transparent conductive film to be a picture element electrode is formed by sputtering and processed into a predetermined pattern by a photolithography step, a picture element electrode 79 and a drain electrode 77 are connected, and thereby an active matrix wafer comprising the color filters formed on the TFT can be formed.
However, according to the investigation into this method by the inventor of the present invention, for the purpose of increasing shading characteristics of a resin black matrix, when a resin black matrix material having a high OD (optical density) value, in detail, an OD value of 3 or more is used, or when the film thickness of the resin black matrix is made thick, in detail, 1.2 μm or more, a problem is produced in that alignment marks for an exposure step cannot be detected. This is because with a high OD value, an exposure alignment laser is absorbed by the resin black matrix and the reflected light from the alignment marks cannot be detected, and with a thick resin black matrix, a step due to the alignment marks are flattened and diffracted light of the exposure alignment laser cannot be detected. Also, for a green filter and blue filter, when the film thickness of the resin black matrix is made thick, in detail 1.2 μm or more, a problem is produced in that the exposure alignment laser is absorbed in the exposure step, whereby the alignment marks cannot be detected.
As a method for forming a color filter wafer on an active matrix wafer without being effected by the abovementioned problems, a method for forming a black matrix not by means of a resin black matrix but by a metal shading film is suggested in Japanese Laid-open Patent Publication No. 122824 of 1996, which is a third prior art. The third prior art is now described in detail with reference to FIGS. 12A to 12C and 13A to 13C.
As shown in FIG. 12A, a channel protection type TFT 10b is formed on a transparent insulated wafer 101, and thereon a passivation film 108 is covered.
Then, as shown in FIG. 12B, a contact through hole 111 for an electrical connection is provided on the passivation film 108. Thereon, a metal film having shading characteristics such as Mo, Cr, Ti, and Al, etc. are formed to the 50 to 1000 nm in thickness by sputtering, etc. and which is patterned into a predetermined shape so as to be a black matrix 115′. At this time, overlapping a black matrix 115 with base wiring is important and alignment between a mask 121 and active matrix wafer 110 for forming a black matrix 115′ is performed as shown in FIG. 14B. In this case, since the black matrix 115 is made of a metal film, when alignment marks are read for alignment, they are recognized not by refracted light but by diffracted light based on the step of the alignment marks. For alignment marks, drain layer alignment marks 123b are used.
Then, as shown in FIG. 12C, a dispersed pigment type photosensitive red resist 113a is coated to be approximately 1.2 μm in thickness by spin coating and a red filter 113a′ is formed into a predetermined pattern by a photolithography step. At this time, alignment between a mask 121 and active matrix wafer 110c for forming a red filter 113a′ is performed as shown in FIG. 14C. For alignment marks, the drain layer alignment marks 123b are used. Since the red resist 113a hardly absorbs an exposure alignment laser (He—Ne) used for reading the alignment marks, the alignment marks can be read by means of a light reflected from the drain layer alignment marks 123b regardless of the film thickness of the red resist 113a. 
Then, as shown in FIG. 13A, for the purpose of forming a green filter 113b′, a dispersed pigment type photosensitive green resist 113b is coated to be approximately 1.2 μm in thickness by spin coating and the green filter 113b′ is formed into a predetermined pattern by a photolithography step. At this time, alignment between a mask 121 and active matrix wafer 110c for forming a green filter 113b′ is performed through the green resist 113b as shown in FIG. 15A. For alignment marks, the drain layer alignment marks 123b are used.
Then, as shown in FIG. 13B, for the purpose of forming a blue filter, a dispersed pigment type photosensitive blue resist 113c is coated to be approximately 1.2 μm in thickness by spin coating and a blue filter 113c′ is formed into a predetermined pattern by a photolithography step. At this time, alignment between a mask 121 and active matrix wafer 110c for forming a blue resist is performed through the blue resist 113c as shown in FIG. 15B. For alignment marks, the drain layer alignment marks 123b are used.
Then, as shown in FIG. 13C, an overcoat layer 114 for flattening the TFT wafer is formed to be approximately 3 μm in thickness on a thin film transistor wafer on which a black matrix 115′, a red filter 113a′, a green filter 113b, and a blue filter 113c′ are formed. For the overcoat layer, a photosensitive acrylic resin is used and after the photosensitive acrylic resin is coated by spin coating, an aperture is provided for a contact through hole portion 161 on the overcoat layer by a photolithography step. Lastly, a transparent conductive film to be a picture element electrode 109 is formed by sputtering, which is processed into a predetermined pattern by a photolithography step, and the picture element electrode 109 and drain electrode 107 is connected.
By the abovedescribed method, a liquid crystal display having an active matrix wafer in which color filters are formed on the TFT can be produced. However, by the method according to the third prior art, since a metal shading film is used for the black matrix, an indoor light made incident from the opposite wafer side is reflected by the metal shading film, and thus there is a problem in that preferable display characteristics cannot be provided. In addition, since the conductive film is formed on a TFT and wiring, there is a problem in that a capacitance combination occurs. Compared with these methods for forming color filters on a TFT layer by a photolithography step, as a fourth prior art, a method for forming color layer on a TFT by an electrodeposition method is suggested, for example, in Japanese Laid-open patent No. 72473 of 1995. Such manufacturing method using electro-deposition is described with reference to FIG. 16.
First, as shown in FIG. 16A, on a transparent insulated wafer 121, a polycrystal silicon film 153, a gate insulating film 123, a gate electrode 122a, an interlayer insulating film 151, source wiring 126b, and a base electrode 152, etc. are intensively formed by a semiconductor process. Then, an area other than the base electrode 152 is covered by a resist 137a. The area thus covered includes a drain side contact portion 141 of the TFT.
Then, as shown in FIG. 16B, the source wiring 126b corresponding to a green picture element is electrically selected and subjected to a electrodeposition treatment, and thereby a green filter 133b′ made of a green electrodeposition film is formed in accordance with the base electrode 152. According to the electrodeposition treatment, an object to be coated is soaked in a container containing an electrodeposition solution colored in green, a direct current is supplied between the object and a counter electrode under appropriate conditions, and thereby forming a colored electrodeposition film on the object to be coated. The electrodeposition film once formed loses its conductivity when prebaking is performed. The electrodeposition solution is an aqueous solution or an aqueous dispersant of a high-polymer resin in which a coloring pigment is dispersed, and of which, for example, an anion type wherein polyester resin having a carboxyl group is neutralized by organic amine can be used. As for the coloring agent, organic pigments are used and are accurately dispersed, thereby securing the quality of the color filters.
Then, as shown in FIG. 16C, source wiring which corresponds to a red picture element is electrically selected and soaked in a red electrodeposition solution, thereby forming a red filter 133a′. Here, since the green electrodeposition film previously formed has lost conductivity due to prebaking, there is no fear of a red filter overlapping thereon. In a similar manner, a blue colored electrodeposition film is also formed on a corresponding picture element area. At the stage where all three primary color filters of red, green, and blue (R, G, and B) have been formed, a main firing is performed.
Then, as shown in FIG. 17A, a used-up resist 137a is peeled and a drain side contact portion 141 of the base electrode 152 is exposed and aligned with each color filter, whereby a picture element electrode 129 is formed by patterning. The picture element electrode 129 is electrically connected to a drain electrode 127 of the TFT via the drain side contact portion 141.
Then, as shown in FIG. 17B, a black matrix 135′ is partially formed by means of the RGB color filters as its shading film by a back exposure method. The back exposure method utilizes the RGB color filters as a shading film from ultraviolet light and the black matrix 135′ is provided on a main wafer in alignment with a gap portion between the RGB color filters. However, the black matrix is not provided on source wiring 126b with shading characteristics.
Lastly, as shown in FIG. 17C, in order to flatten the main wafer, all source wiring in the selected condition are soaked in a black electrodeposition solution and another black matrix 155′ is deposited on the source wiring 126b. 
According to the aforementioned method, a liquid crystal display having an active matrix wafer in which color filters are formed on a TFT by an electrodeposition method can be produced. However, when the color filters are formed on the TFT by the electrodeposition method, routed wiring for supplying the source wiring with a current is necessary, and thus there are problems in that freedom of design is. substantially limited and, therefore, the method is not suitable for production of a highly precise TFT.
The problems in the manufacturing method for an active matrix wafer according to the prior art, which has been described above, are summarized as follows.
First, according to the first prior art, when lamination accuracy of the color filter wafer and active matrix wafer is taken into consideration, a large shading area of the black matrix has to be secured, and therefore, there are problems in that the aperture ratio of the liquid crystal display is decreased and backlight is not effectively utilized.
Then, according to the second prior art, when a resin black matrix material having a OD of 3 or more is used or when the film thickness of the resin black matrix is made thick, a problem is produced in that alignment marks for an exposure step cannot be detected. This is because with a high OD value, an exposure alignment laser is absorbed by the resin black matrix and the light reflected from the alignment marks cannot be detected, and with a thick resin black matrix, a step of the alignment marks are flattened and a diffracted light of the exposure alignment laser cannot be detected. Also, for a green filter and blue filter, when the film thickness of the resin black matrix is made thick, a problems is produced in that the exposure alignment laser is absorbed in the exposure step, whereby the alignment marks cannot be detected.
Then, according to the third prior art, since a metal shading film is used for the black matrix, an indoor light made incident from the opposite wafer side is reflected by the metal shading film, and thus there is a problem in that preferable display characteristics cannot be provided. In addition, since the conductive film is formed on a TFT and wiring, there is a problem in that a capacitance combination occurs.
Lastly, according to the fourth prior art, when the color filters are formed on the TFT by the electrodeposition method therein described, routed wiring for supplying the source wiring with a current is necessary, and thus there are problems in that freedom of design is substantially limited and, therefore, the method is not suitable for production of a highly precise TFT.